Field of the Invention
The present invention relates to amplifiers, and in particular although not exclusively to low-noise radio frequency amplifiers.
Description of the Related Art
The performance of a radio frequency receiver is limited by two conflicting requirements:
1) The noise level generated in the receiver itself, which dictates the minimum received signal level that will meet a given signal to noise ratio. PA1 2) The ability of the receiver to receive weak signals in the presence of much larger signals at frequencies close to that of the weak signals.
Adjacent channel rejection is conventionally achieved in the IF filters of a receiver, so that any front-end amplifying stage, prior to the first frequency changer, may have to cope with large adjacent-channel signals and at the same time introduce the minimum possible level of noise.
If such front-end amplifying stages were perfectly linear they would be able to cope with any level of adjacent-channel signal. However, in practice, such stages are not perfectly linear, and can give rise to intermodulation products that may fall within the wanted channel, contributing to the noise and interfering with the reception of weak signals in the wanted channel. Such amplifier stages are also limited by the maximum signal level that can be handled without compression or clipping occurring.
Conventional methods of improving the linearity of front-end amplifying stages involve the use of series and shunt feedback together with as much open-loop gain as can be generated without exceeding acceptable limits of stability and while retaining a suitable time domain response. The feedback is usually effected by way of resistors, which add noise to the amplifier and result in a degraded noise figure. The effect can be minimised by using small values of resistance, but this then requires high power levels if the advantages of linearity and large signal handling are to be retained.